This invention relates to an improved ranging system including an improved digital single shot circuit that is gateable, non-blocking and retriggerable.
In previous ranging systems such as the last reply ranging system of U.S. Pat. No. 3,545,861, which utilizes the equivalent of a timing counter and a readout counter, the accuracy of the last target determination is limited as a result of the employment of an analog single shot circuit to reset the system's range counter. In that system, the range counter is reset each time a signal representative of a target return signal is applied thereto, to produce the complement of the target range when a control counter reaches its maximum count. If the last target return was received by the above referenced system during a portion of the interval that the range counter was resetting, the return would not be counted by the control counter. In the improved last reply ranging system of the invention, any target return received by the system will reset the timing counter because the improved digital single shot circuit which includes a shift register, provides a delay between the last target return and the termination of the reset pulse that is a selected number of clock pulses representative of the number of stages of the shift register. For purposes of this invention, a digital single shot circuit is a circuit in which a triggering pulse initiates one complete cycle of conditions after the occurrence of a given number of clock pulses, ending with a stable condition.